The performance of a transistor forming a semiconductor device is dependant on its speed, driving current, and leakage current. In an effort to improve the performance of the transistor, (that is, to increase its speed and to reduce its driving and leakage currents), much effort has been made to reduce the resistance of the source/drain regions and the resistance of the gate electrode. For example, a common method used in recent times to reduce these resistances is to form a metal silicide layer on an interfacial surface of the source/drain region and/or on an upper interfacial surface of the gate electrode.
Among the different types of metal suicides, nickel silicide is often used because of its particular advantages with respect to leakage current and level of resistance.
Conventional techniques using a nickel salicidation process are described in U.S. Pat. Nos. 6,383,880 and 6,586,333.
In a conventional method of forming a gate electrode using nickel silicide, nickel (Ni) is first deposited on a silicon substrate having a predetermined gate electrode and source/drain region. As a result, a nickel layer is formed on the interfacial surface of the source/drain region and on an upper interfacial surface of the gate electrode.
Next, a protective metal layer (e.g., a titanium layer or a titanium nitride layer) is formed on the nickel layer. These elements are then heat treated to induce a reaction between the nickel and the silicon, and to thereby form a nickel silicide layer.
However the above described conventional method has a serious flaw. In particular, as a result of the large reaction between the nickel and the silicon, the nickel silicide layer grows abnormally on part of the interfacial surface of the gate electrode and/or on part of the source/drain regions. That is, heat treating is performed at a high temperature of 300-600° C. At this high temperature, the nickel and silicon react too quickly on part of the interfacial surface and/or on part of the source/drain regions. This excessively quick reaction results in abnormal growth of the nickel silicide layer.
As a result of this abnormal growth, the nickel silicide layer has an uneven thickness. This uneven thickness ultimately causes an increase in the leakage current such that the characteristics and driving of the semiconductor device become unstable.